Cholesterol/Phospholipid interactions play a role in cell membrane fluidity, in lung surfactant and in the development of atherosclerosis. Based on pure and mixed monomolecular film studies, our group has proposed the insertion of cholesterol in a monolayer or bilayer results in the formation of two boundary lipid layers around each cholesterol, the inner layer tightly bound, the outer layer only partially affected. It is also proposed that an optimal molecular packing exists where cholesterol has a hydrocarbon chain coordination number of three at which point maximal condensation is achieved. The objective of work yet to be carried out will be to evaluate the apparent uniqueness of cholesterol in terms of its molecular structure and shape. This will be done by detailed extended compositional steroid/phospholipid studies using primarily classical film-balance techniques for the monolayer and differential scanning calorimetry for the bilayer.